1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus, and more specifically to a solid-state image pickup apparatus for reading out signal charges from the photo-sensors of an image pickup section by applying a high voltage to read-out gates of the image pickup section to cause impact ionization to occur to thereby multiply the signal charges.
2. Description of the Background Art
Conventionally, in solid-state image sensors such as a CCD (Charge-Coupled Device) type of image sensors, photo-sensors such as photodiodes photoelectrically convert incident light to corresponding signal charges, vertical and horizontal transfer paths transfer the signal charges vertically and horizontally, respectively, and an output amplifier converts the signal charges to voltage signals to output the latter. Normally, each of the read-out gates is provided between corresponding one of the photo-sensors and the vertical transfer path associated therewith. A solid-state image pickup apparatus with such a solid-state image sensor feeds a timing gate pulse to the read-out gates to send out the signal charges stored on the photo-sensors to the vertical transfer paths.
In interline transfer CCDs widely used in digital cameras, a high positive voltage pulse is normally applied to the read-out gates to read out signal charges from the photo-sensors. Because of an increase of the number of pixels with higher pixel density, the read-out gates have finely been fabricated. Therefore, in the read-out gates, the value of an electric field defined a potential difference per distance becomes extremely great, so that a strong electric field is prone to occur. The photo-sensor and the read-out gate normally form a p-n junction in between. If a strong electric field occurs across the p-n junction, impact ionization will occur.
If the impact ionization specific to each pixel occurs as set forth above, the signal charge read out through the read-out gate will be multiplied, and consequently, advantages such as high picture quality by a common amplifier of CCDs will be ruined. Because of this, there are known techniques for avoiding the impact ionization.
For example, in a solid-state image pickup apparatus disclosed in Japanese patent laid-open publication No. 340099/1996, the interface between a charge storage of an N-type impurity diffusion layer and a read-out channel of a P-type impurity diffusion layer has another N-type impurity diffusion layer formed with an impurity density lower than the charge storage, whereby a potential near the p-n junction between the charge storage portion and the read-out channel is relaxed gradually. Therefore, even if, near the p-n junction, there is a part where dark current occurs, there is no possibility that a phenomenon close to avalanche multiplication will take place. That is, impact ionization due to a strong electric field is suppressed.
However, for use in image pickup apparatuses, there are cases where signal charge obtained as a result of picking up an image of a subject field is small. In such cases, the noise of the output amplifier itself can be a predominant factor of the signal-to-noise ratio. Hence, there are known techniques that utilizing the impact ionization so as to multiply the signal charges prior to the output amplifier to thereby attain a high signal-to-noise ratio.
For instance, in a photoelectric conversion apparatus taught in U.S. Pat. No. 5,528,059 to Isogai, drive means applies a pulse signal to the gate or drain electrode of a J-FET image sensor to drive the apparatus. By making the gate and drain voltages high, a depletion layer between the drain region and the channel end portion forms a high electric field region, so that impact ionization occurs and electrons flowing through the drain region are multiplied. Thus, before amplification by the output amplifier, signal charges are multiplied, whereby the signal-to-noise ratio can be improved.
These techniques are effective in the case where the predominant component of dark noise, i.e. noise under optically shielding, is caused by the amplifier. In the case where the dark current occurs in the vertical transfer paths to cause the dark noise to be worsened, such driving is performed in order to reduce the dark current that the charge transfer capacity of the vertical transfer paths is reduced.
For example, in an image pickup apparatus disclosed by Japanese Patent laid-open publication No. 2005-286470, when setting low-sensitivity and high-sensitivity pickup modes, a solid-state image sensor is driven in first and second read-out modes, respectively. In the first read-out mode, there is a large quantity of incident light, so that the potential well capacity is reduced. In the second read-out mode, there is a small quantity of incident light, so that the potential well capacity is increased. In this apparatus, by changing the signal-charge transfer capacity according to the quantity of light, a high saturated charge quantity and low dark current are compatible with each other.
However, as set forth in the aforementioned U.S. patent to Isogai, when signal charges are multiplied using the impact ionization, it is necessary to increase the transferable capacity of the vertical transfer path, so that dark current cannot be reduced any longer by means of the technique disclosed by the aforementioned '470 Japanese publication.
In the aforementioned '470 publication, for the purpose of reducing the capacity of a potential well that stores an electric charge in the vertical transfer path, the number of vertical transfer electrodes used is reduced, thus merely reducing the dark current from the surface.
In conventional image pickup apparatuses making use of the impact ionization, in the image pickup conditions in which there is a large quantity of dark current, noise will be increased by the impact ionization.
If individual image sensors have different property from each other, they are often different in multiplication factor of signal charge by impact ionization from sensor to sensor. In such a case, when the potential wells of a vertical transfer path are uniform in capacity between the image sensors connected thereto, there are cases, even under the same imaging condition, where signal charges produced by one image sensor can be satisfactorily transferred over the vertical transfer path, but signal charges produced by another image sensor will overflow the potential well connected to the other image sensor.